Various members for a valve are used in various high temperature and high pressure lines and chemical solution lines. For example, a considerably high strength is required when a case body of a valve drive section and a valve body of a butterfly valve are used at high temperature under high pressure. Therefore, a member for a valve made of metal such as iron or aluminum has hitherto been used. However, as the member for a valve is sometimes used in applications (exposed outdoors, in a coastal zone, in a seawater environment) which require corrosion resistance and weatherability for a long period, poor operation may occur in a case body of the valve drive section made of metal and obstruction of opening and closing of a valve may occur in a valve body of a butterfly valve made of metal, and thus there arises a problem that long-period stability deteriorates. When used in various chemical solution lines, there arises a problem that leakage of a chemical solution is caused by erosion with a corrosive gas or a chemical solution in a member for a valve made of metal, particularly a butterfly valve body made of metal, and that some members for a valve sometimes cannot be used according to the kind of the chemical solution.
To solve these problems, the case body of the valve drive section made of metal was coated with an epoxy resin, while chemical resistance and corrosion resistance were improved by forming a resin or rubber lining in the valve body of the butterfly valve made of metal.
However, the following problems arose in the case body of the valve drive section made of metal coated with an epoxy resin.
1. As an epoxy resin is a very brittle resin, a coating film of the epoxy resin may be peeled off with ease when impact or stress is directly applied during transportation or use.
2. Even if impact or stress is not directly applied to the coated surface, when used in the environment with extremes of temperature, the valve drive section made of metal and the coated surface differ in an expansion coefficient and thus the coated surface may be peeled off with ease by repetition of thermal expansion and thermal contraction.3. At the position where the coating film is peeled off, deterioration due to corrosion or poor operation may occur as in the case of the member for a valve made of metal which is not coated.4. As the weight of the case body accounts for half or more of the weight of the valve drive section, the case body of the valve drive section made of metal is very heavy and thus it is difficult to carry and the workability becomes inferior.
In the valve body of the butterfly valve made of metal on which a resin or rubber lining is formed, similar to the coating of the above items 1 and 2, the lining can be peeled off. Similar to the above item 3, there can arise a problem that corrosion occurs at the position where the lining was peeled off. Similar to the above item 4, there can arise a problem that the case body becomes very heavy. Although the resin or rubber lining is not as brittle, compared with the epoxy resin, a thin lining is formed and therefore the lining may be peeled off when impact or stress is directly applied during transportation or use.
As means for solving the above problems, there has been proposed a method of forming a casing of a gear drive unit as a case body with a heat transfer and conducting synthetic resin (see, for example, Patent Document 1 (Japanese Unexamined Patent Publication (Tokukai) No. 7-151263, pages 1 to 4 and FIG. 1)). According to this method, the casing of the gear drive unit for valve is formed of a synthetic resin such as rigid vinyl chloride, epoxy or FRP and, even when used for a piping material through which a low temperature fluid flow, dew condensation occurs on the internal and external surfaces of the casing and a gear mechanism in the casing does not occur and therefore water droplets are not formed, and thus obstruction in operation due to rust can be prevented.
In the valve body of the butterfly valve, there is proposed a method of forming a valve box as the valve body with a fiber-reinforced resin (see, for example, Patent Document 2 (Japanese Unexamined Patent Publication (Tokukai) No. 6-288478, pages 1 to 5 and FIG. 4)). According to this method, a valve box of a butterfly valve made of a resin is formed by disposing a fiber-reinforced prefoam in a ring-shaped flask using a fiber-reinforced resin such as FRP, charging a resin solution while rotating the ring-shaped flask, passing the resin solution through the reinforcing fiber, and transferring outside thereby to gradually increase the thickness inside. Thus, there can be formed a valve box which is hardly corroded even if scratched by an external force or fluid pressure, and also has improve durability.